Implantable neurostimulation systems have proven therapeutic in a wide variety of diseases and disorders. Pacemakers and Implantable Cardiac Defibrillators (ICDs) have proven highly effective in the treatment of a number of cardiac conditions (e.g., arrhythmias). Spinal Cord Stimulation (SCS) systems have long been accepted as a therapeutic modality for the treatment of chronic pain syndromes, and the application of tissue stimulation has begun to expand to additional applications such as angina pectoralis and incontinence. Deep Brain Stimulation (DBS) has also been applied therapeutically for well over a decade for the treatment of refractory chronic pain syndromes, and DBS has also recently been applied in additional areas such as movement disorders and epilepsy. Further, in recent investigations, Peripheral Nerve Stimulation (PNS) systems have demonstrated efficacy in the treatment of chronic pain syndromes and incontinence, and a number of additional applications are currently under investigation. Furthermore, Functional Electrical Stimulation (FES) systems, such as the Freehand system by NeuroControl (Cleveland, Ohio), have been applied to restore some functionality to paralyzed extremities in spinal cord injury patients.
Each of these implantable neurostimulation systems typically includes one or more electrode carrying stimulation leads, which are implanted at the desired stimulation site, and a neurostimulator implanted remotely from the stimulation site, but coupled either directly to the stimulation lead(s) or indirectly to the stimulation lead(s) via a lead extension. Thus, electrical pulses can be delivered from the neurostimulator to the stimulation lead(s) to stimulate or activate a volume of neural tissue in accordance with a set of stimulation parameters and provide the desired efficacious therapy to the patient. A typical stimulation parameter set may include the electrodes that are sourcing (anodes) or returning (cathodes) the stimulation pulses at any given time, as well as the magnitude, duration, and rate of the stimulation pulses. The neurostimulation system may comprise a handheld patient programmer to remotely instruct the neurostimulator to generate electrical stimulation pulses in accordance with selected stimulation parameters. The handheld programmer may, itself, be programmed by a technician attending the patient, for example, by using a Clinician's Programmer Station (CPS), which typically includes a general purpose computer, such as a laptop, with a programming software package installed thereon.
The best stimulus parameter set will typically be one that provides stimulation energy to the volume of neural tissue that must be stimulated in order to provide the therapeutic benefit (e.g., pain relief), while minimizing the volume of non-target neural tissue that is stimulated. However, because the target neural tissue (i.e., the tissue associated with the therapeutic effects) and non-target neural tissue (i.e., the tissue associated with undesirable side effects) are often juxtaposed, therapeutically stimulating neural tissue while preventing side effects may be difficult to achieve.
For example, in SCS, stimulation of the spinal cord creates the sensation known as paresthesia, which can be characterized as an alternative sensation that replaces the pain signals sensed by the patient. To produce the feeling of paresthesia without inducing involuntary motor movements within the patient, it is often desirable to preferentially stimulate nerve fibers in the dorsal column (DC nerve fibers), which primarily include sensory nerve fibers, over nerve fibers in the dorsal roots (DR nerve fibers), which include both sensory nerve fibers and motor reflex nerve fibers. While DC nerve fibers are the intended targets in conventional SCS, in fact, the DR nerve fibers often are recruited first because of geometric, anatomical, and physiological reasons. For example, the DR nerve fibers have larger diameters than the largest nearby DC nerve fibers, and thus, have a lower threshold at which they are excited. Other factors that contribute to the lower threshold needed to excite DR nerve fibers are the different orientations of the DC nerve fibers and DR nerve fibers, the curved shape of the DR nerve fibers, and the inhomogeneity and anisotropy of the surrounding medium at the entrance of the DR nerve fibers into the spinal cord. Thus, DR nerve fibers may still generate action potentials at lower voltages than will nearby DC nerve fibers. As a result, the DC nerve fibers that are desired to be stimulated have a lower probability to be stimulated than do the DR nerve fibers, and thus, the reflex motor nerve fibers intermingled among the sensor nerve fibers of a dorsal root are often recruited, leading to discomfort or muscle twitching, thereby preventing satisfactory paresthesia coverage.
For reasons such as this, it is often desirable to modify the threshold at which neural tissue is activated in a manner that maximizes excitation of the target neural tissue, while minimizing excitation of the non-target neural tissue. This can be accomplished by applying depolarizing sub-threshold conditioning pulses (or pre-pulses) to render neural tissue (and in this case, the non-target neural tissue) less excitable to the subsequent stimulation pulse and/or applying hyperpolarizing sub-threshold conditioning pulses to render tissue (and in this case, target neural tissue) more excitable to the subsequent stimulation pulse.
Pre-pulsing was designed in the context of monopolar stimulation; that is, monopolar pre-pulses followed by monopolar stimulation pulses. Subsequent conditioning arrangements have contemplated the use of multipolar pre-pulses followed by multipolar stimulation pulses for SCS and DBS applications. For example, as shown in FIG. 1, it is known to place three electrodes 1a-1c in contact with a spinal cord SC along a line that is transverse to the axis of the spinal cord SC, so that the center electrode 1b is located at the center of the DC nerve fibers, and the two outer electrodes 1a, 1c are located adjacent the DR nerve fibers extending from the spinal cord SC.
Tripolar conditioning energy, and then tripolar stimulation energy, is conveyed from the electrodes 1a-1c in accordance with a pulse pattern that preferentially stimulates the DC nerve fibers, while inhibiting the stimulation of the DR nerve fibers. In particular, as shown in FIG. 2, during a conditioning period, depolarizing, sub-threshold, cathodic pre-pulses 2 are respectively conveyed from the outer electrodes 1a, 1c to render the DR nerve fibers less excitable, while a hyperpolarizing, sub-threshold, anodic pre-pulse 4 is conveyed from the center electrode 1b to render the DC nerve fibers more excitable. During a stimulation period, anodic pulses 6 are then conveyed from the outer electrodes 1a, 1c, and a cathodic stimulation pulse 8 is conveyed from the center electrode 1b. Because the DR nerve fibers have been rendered less excitable by the depolarized pre-pulses 2, the subsequent anodic pulses 6 will not stimulate the DR nerve fibers. In contrast, because the DC nerve fibers have been rendered more excitable by the hyperpolarizing pre-pulse 4, the subsequent cathodic stimulation pulse 8 will stimulate the DC nerve fibers.
While coupling monopolar conditioning pulses with monopolar stimulation pulses, and coupling multipolar conditioning pulses with multipolar stimulation pulses, has proven successful in preferentially stimulating nerve fibers, there are certain benefits to monopolar conditioning and stimulation over multipolar conditioning and stimulation, and vice versa. Thus, the benefits of coupling conditioning pulses and stimulation pulses may not be fully maximized.
There, thus, remains a need for an improved neurostimulation method and system that couples conditioning pulses with stimulation pulses.